This invention relates to the art of electrostatographic copying and more specifically to a novel method for the preparation of a photosensitive device.
In the art of electrostatographic copying, a plate comprising a photoconductive insulating layer is electrostatically charged in the dark in order to apply a uniform charge to its surface. The charged plate is then exposed to activating radiation in imagewise configuration to selectively dissipate the charge in the illuminated areas while leaving behind a latent electrostatic image corresponding to the non-illuminated areas. The latent image is then developed by depositing finely divided electroscopic marking materials on the surface of the plate. This concept, which was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, has been further amplified in many related patents in the field.
Conventional xerographic plates usually comprise a photoconductive insulating layer overlaying a conductive substrate. A photoconductive material which has been widely used as a reusable photoconductor in commercial xerography comprises amorphous selenium.
An improved type of photoreceptor useful in electrostatographic copying comprises an electrically conductive substrate having on its surface a relatively thin layer of light absorbing photoconductive charge carrier generating material overcoated with a relatively thick layer of substantially transparent, organic, active transport material. This type of photoreceptor is advantageous due to its increased flexibility and the protection from physical damage afforded the photoconductor by the overlayer of active transport material. In addition, this configuration facilitates the use as carrier generators of photoconductive materials which are too conductive in the dark for use in conventional single layer photoreceptors. Another type of photoreceptor which has come into use in recent years comprises a conductive substrate having a layer of photoconductive material on its surface which is overcoated with an electrically insulating organic material.
The usual method of preparing a photoreceptor having a layer of photoconductive material overcoated with an organic material (either active transport or insulating) involves applying the layer of photoconductive material to the substrate, such as by vapor deposition of selenium, and then applying a solution of the organic overcoating material to the photoconductor surface. Evaporation of the solvent, which is normally carried out at an elevated temperature, leaves an adherent, continuous layer of the organic overcoating material.
This two step procedure has several drawbacks. First of all, it is relatively cumbersome and expensive since two separate coating steps are needed, each requiring separate control of deposition and coating thickness. Furthermore, the second coating step must be such as not to disturb the first layer, e.g. by re-solution, re-crystallization or mechanical disturbance thereof. Since the second coating material must be applied from a solvent which does not disturb the first layer, it is difficult to avoid formation of an electrical barrier between the dielectric layers, which barrier may act to impede the free flow of electrical charge carriers between these layers. In addition, since an additional interface is present, there may be considerable difficulty in adhering the final coating to the photoconductive sub-coating.
It would be desirable and it is an object of the present invention, to provide a novel method for the preparation of an electrostatographic photoreceptor comprising a conductive substrate, a layer of a photoconductive material and an overcoating layer of an active transport or insulating organic material.
It is another object to provide a novel single step process for applying two layers to a conductive substrate in a well-controlled manner.
It is another object to provide a coating process for applying simultaneously a photoconductive pigmented carrier generator layer dispersed in an "active" organic binder and an overlaying "active" transport layer consisting essentially of the same "active" binder material without a significant proportion of the pigmentary photoconductive material.
A further object is to provide a coating process for the simultaneous deposition of a mixture of two or more photogenerating photoconductive pigments in a single layer overcoated by an active overcoating.
An additional object is to provide a process for the simultaneous deposition of a relatively thin photoconductor-binder layer and a relatively thick photodischargeable protective layer therefore.
A further object is to provide a simplified fabrication method for the double layer photoconductor/dielectric photoreceptors used for the charge-recharge processes of Hall, Canon and Katsuragawa as described in Photographic Science and Engineering; Vol. 18, No. 3, May/June 1974 (pp. 254-261).